Method and apparatus for treating waste gas containing PFC and/or HFC

ABSTRACT

A method for treating waste gas containing PFC and/or HFC, comprising contacting a mixture of gas waste containing PFC and/or HFC, ozone, and water with an iron oxide catalyst at a temperature between 50 and 300° C. by gas-solid contact to perform an oxidation reaction for reducing the amount of PFC and/or HFC. An apparatus for treating waste gas containing PFC or HFC is also provided. The operational temperature in the present invention is much lower than the prior art, and thus provides lower energy consumption and little risk of fire. The present invention is suitable for the treatment of waste gas containing PFC and/or HFC, especially for the removal of perfluorocompounds from the waste gas generated by semiconductor and photoelectrical product manufacturing plants.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and apparatus fortreating waste gas and in particular to a method and apparatus fortreating waste gas containing PFC and/or HFC.

[0003] 2. Description of the Related Art

[0004] In perfluorinated compound (PFC) waste reduction technology,process modification, such as promoting the utilization efficiencies ofperfluorinated compounds in the process and using substitutes forperfluorinated compounds, recover and reuse, and end-of-pipe treatmentare commonly used. Although end-of-pipe treatment is not the best choicefor waste reduction, it is the most mature, economical, and efficientway to reduce perfluorinated compounds and/or hydrofluorocarbons (HFCs).Among the various end-of-pipe treatment methods, the combustion-baseddecomposition process is a well-developed PFC treatment technology,which uses a combustion temperature higher than 1200° C. The catalyticcombustion-based decomposition process treats PFCs with catalysts at arelatively low temperature between about 500° C. and 750° C. Theplasma-based process utilizes a localized, relatively high temperatureto treat PFC and the central temperature may exceed than 3000° C. Thesemethods consume an excessive amount of energy and pose a high risk offire.

[0005] U.S. patent application Ser. No. 20020150527 and WO 01/21304 A1disclose a catalyst composition and a treatment method wherein PFC andHFC are decomposed using the catalyst composition. The catalystcomposition includes aluminum oxide preferably stabilized by theaddition of, for example, titanium, zirconium, cobalt, compoundsthereof, or mixtures thereof. The operating temperature must be higherthan 300° C., and preferably from about 500° C. to 800° C., without theuse of ozone and iron oxide.

[0006] Among transitional metal oxides, ferric oxide has moderateoxidation reactivity respective to H₂, CO, CH₄, and paraffin. Theexamples of transitional metal oxides as catalysts are mainly seen inapplications for oxidation, hydrogenation, or dehydrogenation. Ferricoxide is commonly used as a catalyst in, for example, high temperatureconversion reaction, synthesis reaction of polystyrene from styrene,ammonia synthesis reaction, and reaction for removal of hydrogensulfide.

[0007] Ozone is a very strong oxidant and widely used in watertreatment, organic synthesis, and food sanitizing. Upon thedecomposition of the pollutant by ozone, the ozone is immediatelyreduced to molecular oxygen without generating residue or causingsecondary pollution.

[0008] Nevertheless, iron oxide and ozone have never been used incombination for treating PFCs and HFCs.

[0009] In view of the United Nations Framework Convention on ClimateChange and Kyoto Protocol, and the parties agreeing to the Protocol, theemissions of greenhouse gas (including PFCs and HFCs) are to be furtherrestricted between 2008 and 2012 to protect the environment. Hence,there is a need for a better method and apparatus for treating waste gascontaining PFC and/or HFC.

SUMMARY OF THE INVENTION

[0010] Accordingly, to resolve the problems of excessive energyconsumption and the high risk of fire posed by the conventional methodsmentioned above, an object of the invention is to provide a method andapparatus for treating waste gas containing PFC and/or HFC.

[0011] In order to achieve the above object, the invention provides amethod for treating waste gas containing PFC and/or HFC, which comprisesthe step of contacting a mixture of the waste gas containing PFC and/orHFC, ozone, and water with an iron oxide catalyst to undergo gas-solidcontact and oxidation reaction at a temperature between 50° C. and 300°C. to reduce the amount of PFC and/or HFC.

[0012] The invention further provides a method for treating waste gascontaining PFC and/or HFC, which comprises the steps of (i) heating thewaste gas containing PFC and/or HFC in a concentration of 100 to 50000ppmv, and water in a concentration of 1 to 1000 ppmv, at a temperaturebetween 50° C. and 300° C., (ii) introducing ozone in a concentration of100 to 50000 ppmv, to the resultants from the step (i) and mixing toform a uniform mixture, and (iii) subjecting the mixture of the wastegas containing PFC and/or HFC, ozone, and water and an iron oxidecatalyst to gas-solid contact and oxidation reaction at a temperaturebetween 50° C. and 300° C. for 1.0 to 10 seconds retention time toreduce the amount of PFC and/or HFC.

[0013] The invention also provides an apparatus for treating waste gascontaining PFC and/or HFC, which comprises a heating device for heatingincoming waste gas containing PFC and/or HFC and water at a temperaturebetween about 50 and 300° C., an ozone generator for generating ozone, amixing device for receiving and uniformly mixing the ozone from theozone generator and the waste gas containing PFC and/or HFC and thewater from the heating device to form a gas mixture, and a reactor forreceiving the gas mixture from the mixing device, wherein, the reactorcontains an iron oxidation catalyst and the gas mixture and the ironoxidation catalyst are subjected to gas-solid contact and oxidationreaction at a temperature between about 50 and 300° C.

[0014] Waste gas containing PFC and/or HFC can be treated efficientlyusing the present method and apparatus, especially, when HFC and/or HFCare used as raw material in the semiconductor industry orphotoelectrical industry. Among these compounds, the treatment of C₂F₆is the most difficult, but can be easily accomplished in the presentinvention. Furthermore, in the present invention, PFC and/or HFC reactwith ozone and the catalyst at a controlled temperature less than about300° C., thus eliminating the problems of secondary pollution caused byexhausted ozone-containing gas, excessive energy consumption, and highrisk of fire.

[0015] A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, wherein:

[0017]FIG. 1 is an illustration of an example of the apparatus fortreating waste gas containing PFC and/or HFC of the present invention;

[0018]FIG. 2 is a plot showing the removal efficiency versus thetemperature of an example of the apparatus for treating waste gascontaining PFC and/or HFC of the present invention; and

[0019]FIG. 3 is a plot showing the removal efficiency versus theconcentration of ozone of an example of the apparatus for treating wastegas containing PFC and/or HFC of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is now described in detail with referenceto FIG. 1.

[0021] In the present method for treating waste gas containing PFCand/or HFC, waste gas containing PFC and/or HFC 9 and moisture 10 areheated through a heating device 1. The resulting gas mixture iscontrolled at a temperature between about 50 and 300° C., but not higherthan the 500° C. used in the conventional perfluorinated compoundtreatment technology, thus greatly reducing energy consumption. Theozone (not shown in FIG. 1) generated by the ozone generator 2 and thewaste gas containing PFC and/or HFC 9 and moisture 10 from the heatingdevice 1 are introduced into a mixing device 4, and sufficiently stirredand mixed. Alternatively, the introduction site of ozone may be prior tothe heating device.

[0022] Examples of PFCs to be treated using the present method are CF₄,C₂F₆, C₃F₈, NF₃, SF₆, and a combination thereof. An example of HFC isCHF₃. The loading of the waste gas for treatment may be 100 to 50000ppmv, and preferably 1000 to 10000 ppmv.

[0023] The concentration of incoming moisture 10 (that is, water) may be1 to 1000 ppmv, and preferably 200 to 500 ppmv.

[0024] The concentration of incoming ozone may be 100 to 50000 ppmv, andpreferably 1000 to 10000 ppmv.

[0025] The mixed waste gas containing PFC and/or HFC 9, ozone, andmoisture 10 are introduced into a reactor 3 to undergo gas-solid contactand oxidation reaction with a catalyst in the reactor 3 at a temperaturekept between 50 and 300° C., and preferably between 100 and 200° C. Thecatalyst bed may be vertical or horizontal. Suitable catalysts are ironoxides, including ferric oxide, ferric oxide monohydrate, ferriferrousoxide, ferriferrous oxide monohydrate and a combination thereof,preferably ferric oxide monohydrate, and specifically, needle shapedferric oxide monohydrate which is also known as goethite (FeOOH).

[0026] The retention time for waste gas in the catalyst bed may be 0.1to 10 seconds. It may be more than 10 seconds, but because of economicconsiderations, a shorter amount of time is preferred. The ratio of theconcentrations of PFC and/or HFC to ozone is 0.1 to 10 ppmv/ppmv. Thehumidity at the site before the inlet of the reactor 3 is controlled toremain between 20% and 100%. The humidity herein is relative humidityand came from the process.

[0027] After the above-mentioned waste gas containing PFC and/or HFC 9to be treated is subjected to contact and oxidation reaction with thecatalyst in the reactor 3, the perfluorinated compounds and/or thehydrofluorocarbons contained in the waste gas can be decomposed intoCO₂, H₂O, and inorganic acids which are dischargeable. Preferably, ascrubbing tower 5 with a scrubbing solution is further installed forremoving the inorganic acids produced by the treatment before discharge.

[0028] Optionally, a windmill 8 can be installed prior to the inlet ofthe system of the present invention, or after the outlet of the system,providing the function of gas collection.

[0029] Optionally, the humidity, temperature, and ozone concentration ofthe gas stream at the site prior to the heating procedure of waste gasand water or prior to or after the gas-solid contact procedure aredetected by a humidity, temperature, and ozone concentration monitor andauto-control device 6. The variations of the collected data are comparedand provided for the control of humidity, temperature, ozoneconcentration, and flow rate of the scrubbing solution (if used), inorder to maintain stable, high treatment efficiency of the integralapparatus through the adjustment of these controlling factors,especially in the case of a high concentration or a dramatically variedload of waste gas containing PFC and/or HFC.

[0030] The present invention can be directly applied to combustiontechnology for perfluorinated compound waste gas and the categories andconcentration range of the treatable perfluorinated compounds.Additionally, the operation temperature for the present invention isless than 300° C., thus excessive energy consumption and the risk offire posed by conventional combustion technology are avoided.Furthermore, the present method is a stable treatment for cases wherethe concentration of PFC is high or the load is varied dramatically. Thepresent invention effectively treats a wide range of PFC concentrationsand types.

[0031] Referring to FIG. 1, the manufacture of the apparatus fortreating waste gas containing PFC and/or HFC of the present invention isdescribed as follows. The material for the devices and conduits arepreferably corrosion-resistant and heat-resistant. Because the operationtemperature is less than 300° C. in the present invention, thelimitation of heat-resistance is less stringent than that in the priorart. The material contacting gasses such as waste gas containing PFCand/or HFC and ozone can be selected from, for example, SS316 stainlesssteel, ceramics, quartz, and Teflon.

[0032] Suitable heating devices are conventional heating devices forreceiving and heating the incoming waste gas containing PFC and/or HFC 9and water 10 at a temperature between 50 and 300° C. The externalheating devices are preferred for their convenience as long as thedesired temperature is attainable.

[0033] Suitable ozone generators are common ozone generators which cangenerate ozone from the supply of air or oxygen 11, for generating ozoneneeded in the treatment of waste gas containing PFC and/or HFC 9. Theozone generator 2 can be connected prior to or after the heating device1.

[0034] The mixing device 4 is for receiving and uniformly mixing theozone and the waste gas containing PFC and/or HFC 9 and moisture 10 fromthe heating device 1. Suitable mixing devices are preferably those whichcan stir the gasses to form a uniform gas mixture and have corrosive andheat resistance up to a temperature between 50 and 300° C.

[0035] The reactor 3 contains an iron oxide catalyst and receives thegas mixture from the mixing device 4 and hosts gas-solid contact andoxidation reaction. Suitable iron oxides for use as the catalyst areferric oxide, ferric oxide monohydrate, ferriferrous oxide, ferriferrousoxide monohydrate and a combination thereof, and preferably in particleor powder form. In order to undergo the gas-solid contact and oxidationreaction at a temperature between 50 and 300° C., and preferably between100 and 200° C., the reactor 3 can be further equipped with a thermostator a heating device (not shown in FIG. 1). The iron oxide catalyst bedmay be vertical or horizontal.

[0036] A scrubbing tower 5 may be further connected to the reactor 3 forreceiving the gas from the reactor 3 so that the inorganic acidsproduced from the reactor 3 can be transferred into a scrubbingsolution. The scrubbing solution may be a sodium hydroxide solution. Theflow rate of the scrubbing solution is controlled by a flow controller7. After saturation, the scrubbing solution may be discharged to a wastewater treatment plant.

[0037] Furthermore, a humidity, temperature, and ozone concentrationmonitor and auto-control device 6 may be installed at the site prior tothe inlet of the heating device 1, prior to the inlet of the reactor 3,or after the outlet of the reactor 3, for optimally controllingtreatment conditions. Treatable conditions include, for example,humidity, temperature, ozone concentration, PFC concentration, and theflow rate of the scrubbing solution (if used) to obtain an optimalefficiency of treatment.

[0038] Optionally, a windmill 8 can be installed prior to the inlet ofthe system of the present invention, or after the outlet of the system,providing the function of gas collection.

[0039] The apparatus of the present invention is suitable for treatingwaste gas containing PFC and/or HFC as mentioned above. The examples ofPFC are CF₄, C₂F₆, C₃F₈, NF₃, SF₆, and a combination thereof. Theexample of HFC is CHF₃.

EXAMPLE 1

[0040] The treatment of waste gas containing PFC or HFC was performedusing the apparatus for treating waste gas containing PFC and/or HFC ofthe present invention as shown in FIG. 1. The waste gas to be treatedwas hexafluoroethane, the typical perfluorinated compound gas frequentlyused in semiconductor or photoelectrical manufacturing and also one ofthe most difficult-to-treat PFCs. The catalyst used was needle-shapedferric oxide monohydrate (FeOOH). The incoming concentration ofhexafluoroethane was maintained at a level of about 4000 to 6000 ppmv.The humidity of gas just prior to the inlet of the reactor was 50%. Thetemperature was maintained at 100 to 150° C. P Various concentrations ofozone were used. In the steady state, the retention time for the mixtureof hexafluoroethane, ozone, and moisture in the catalyst bed was 0.5second. The results are shown in Table 1 and FIG. 2. TABLE 1 Results forusing different concentrations of ozone in Example 1 Removed C₂F₆ Ozoneconc. (ppmv) C₂F₆ Conc. (ppmv) Conc. Removal inlet outlet Inlet outlet(ppmv) efficiency % 2600 <1 4200 1500 2700 64 2000 <1 4000 2000 2000 501200 <1 4000 3000 1000 25 5200 <1 6000 1500 4500 83

[0041] As shown in Table 1, the higher the concentration of ozonerelative to the concentration of perfluorinated compound, the better theremoval efficiency for the perfluorinated compound. When theconcentration of ozone was increased to 5200 ppmv, the removalefficiency for hexafluoroethane was improved to about 83%.

EXAMPLE 2

[0042] Example 2 was performed in the same way as Example 1, except thatvarious temperatures were used. The operating conditions in a steadystate were as follows. The incoming concentration of hexafluoroethanewas maintained at about 4000 ppm to 5000 ppmv, the concentration ofozone was 2600 ppmv, the humidity of gas just prior to the inlet of thereactor was 50%, and the retention time was 0.5 second. The results areshown in Table 2 and FIG. 3. TABLE 2 Results for using differentoperation temperatures in Example 2 Ozone Conc. Removed C₂F₆ Temp.(ppmv) C₂F₆ Conc. (ppmv) Conc. Removal (° C.) inlet outlet inlet outlet(ppmv) efficiency % 25 2600 <1 4200 4100 100 2 50-80 2600 <1 4000 30001000 25  80-100 2600 <1 4000 2200 1800 45 100-150 2600 <1 4000 1500 300075

[0043] As shown in Table 2, when the temperature was 25° C., the removalefficiency for hexafluoroethane was near zero. When the temperature waselevated to 100-150° C., the removal efficiency for hexafluoroethaneimproved to 75%.

[0044] While the invention has been described by way of example and interms of the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments. To the contrary,it is intended to cover various modifications and similar arrangements(as would be apparent to those skilled in the art). Therefore, the scopeof the appended claims should be accorded the broadest interpretation soas to encompass all such modifications and similar arrangements.

What is claimed is:
 1. A method for treating waste gas containing PFCand/or HFC, comprising the step of contacting a mixture of the waste gascontaining PFC and/or HFC, ozone, and water with an iron oxide catalystto undergo gas-solid contact and oxidation reaction at a temperaturebetween 50° C. and 300° C. to reduce the amount of PFC and/or HFC. 2.The method as claimed in claim 1, wherein the iron oxide is selectedfrom the group consisting of ferric oxide, ferric oxide monohydrate,ferriferrous oxide, ferriferrous oxide monohydrate and a combinationthereof.
 3. The method as claimed in claim 1, wherein the PFC is CF₄,C₂F₆, C₃F₈, NF₃, SF₆, or a combination thereof.
 4. The method as claimedin claim 1, wherein the HFC is CHF₃.
 5. The method as claimed in claim1, wherein the retention time for the waste gas containing PFC and/orHFC in the bed of the iron oxide catalyst is from 1.0 to 10 seconds, theconcentration ratio of PFC and/or HFC to ozone is from 0.1 to 10, andthe humidity prior to reaction is between 20% and 100%.
 6. The method asclaimed in claim 1, wherein the temperature is between about 100° C. andabout 150° C.
 7. A method for treating waste gas containing PFC and/orHFC, comprising the steps of: (i) heating the waste gas containing PFCand/or HFC in a concentration of from 100 to 50000 ppmv and water in aconcentration of from 1 to 1000 ppmv at a temperature between 50° C. and300° C.; (ii) introducing ozone in a concentration of from 100 to 50000ppmv to the resultants from the step (i) and mixing to form a uniformmixture; and (iii) subjecting the mixture of the waste gas containingPFC and/or HFC, ozone, and water and an iron oxide catalyst to gas-solidcontact and oxidation reaction at a temperature between 50° C. and 300°C. for 1.0 to 10 seconds retention time to reduce the amount of PFCand/or HFC.
 8. The method as claimed in claim 7, wherein the iron oxideis selected from the group consisting of ferric oxide, ferric oxidemonohydrate, ferriferrous oxide, ferriferrous oxide monohydrate and acombination thereof.
 9. The method as claimed in claim 7, wherein thePFC is CF₄, C₂F₆, C₃F₈, NF₃, SF₆, or a combination thereof.
 10. Themethod as claimed in claim 7, wherein the HFC is CHF₃.
 11. The method asclaimed in claim 7, wherein the temperature is between 100° C. and 150°C.
 12. The method as claimed in claim 7, further comprising the step of:(iV) removing inorganic acid produced from step (iii) with a scrubbingsolution.
 13. The method as claimed in claim 7, further comprising,after step (i) or before or after step (iii), the step of: monitoringand controlling the temperature, the moisture, and the concentration ofozone.
 14. The method as claimed in claim 12, further comprising, afterstep (i) or before or after step (iii), the step of: monitoring andcontrolling the temperature, the moisture, and the concentration ofozone.
 15. An apparatus for treating waste gas containing PFC and/orHFC, comprising: a heating device for heating incoming waste gascontaining PFC and/or HFC and water at a temperature between about 50°C. and 300° C.; an ozone generator for generating ozone; a mixing devicefor receiving and uniformly mixing the ozone from the ozone generatorand the waste gas containing PFC and/or HFC and the water from theheating device to form a gas mixture; and a reactor for receiving thegas mixture from the mixing device, wherein, the reactor contains aniron oxidation catalyst and the gas mixture and the iron oxidationcatalyst are subjected to gas-solid contact and oxidation reaction at atemperature between about 50° C. and 300° C.
 16. The apparatus asclaimed in claim 15, wherein the heating device is upstream from theozone generator or the ozone generator is upstream from heating device.17. The apparatus as claimed in claim 15, further comprising a monitorand auto-control device connecting to at least one of the sites beforethe inlet of the heating device, before the inlet of the reactor, andafter the outlet of the reactor, for monitoring and controlling thetemperature, the moisture, and the ozone concentration at the site. 18.The apparatus as claimed in claim 15, wherein the iron oxide is selectedfrom the group consisting of ferric oxide, ferric oxide monohydrate,ferriferrous oxide, ferriferrous oxide monohydrate and a combinationthereof.
 19. The apparatus as claimed in claim 15, wherein the PFC isCF₄, C₂F₆, C₃F₈, NF₃, SF₆, or a combination thereof.
 20. The apparatusas claimed in claim 15, wherein the HFC is CHF₃.
 21. The apparatus asclaimed in claim 15, wherein the temperature is between about 100° C.and 150° C.
 22. The apparatus as claimed in claim 15, furthercomprising: a scrubbing tower for receiving the gas from the reactor totransfer inorganic acids in the gas produced from the reactor to ascrubbing solution.